OBJECTIVES: To measure the risk of preterm delivery, premature rupture of membranes, infant low birth weight and infant mortality, by a population-based retrospective cohort study using Washington State birth certificate data.METHODS: All women diagnosed with Chlamydia trachomatis infection (n = 851), noted with a check box on the birth certificate from 2003, and a randomly selected sample of women not diagnosed with C trachomatis (n = 3404) were identified. To assess the RR between chlamydia infection and pregnancy outcomes, multivariable logistic regression analysis was used.RESULTS: Women with chlamydia infection were younger, more likely to be non-white and had less years of education compared with women without chlamydia. Additionally, they were more likely to have inadequate prenatal care and coinfections with other sexually transmitted infections. After adjusting for age and education, chlamydia-infected women were at an increased risk of preterm delivery (RR 1.46, 95% CI 1.08 to 1.99) and premature rupture of membranes (RR 1.50, 95% CI 1.03 to 2.17) compared with non-infected women. However, no increased risk of infant death (RR 1.02, 95% CI 0.37 to 2.80) or low birth weight (RR 1.12, 95% CI 0.74 to 1.68) associated with chlamydia infection was observed.CONCLUSION: This study suggests that C trachomatis is associated with an increased risk of preterm delivery and premature rupture of membranes, but not with infant death and low birth weight. Routine screening and opportune treatment for C trachomatis should be considered a necessary part of prenatal care to reduce these adverse pregnancy outcomes.